ASU study: Parts of metro Phoenix area are sinking

Ground elevation levels in Apache Junction are seeing the fastest drop, followed by Sun City West, Peoria and the north Valley, ASU researchers say.

The map is color coded so blue areas are where the ground has subsided and red/orange areas are where uplift in the ground has occurred. During the study, subsidence occurred mainly in the metro areas surrounding Phoenix and did not have an observable effect on the majority of the city of Phoenix. Subsidence occurred in much of the northwest valley (Sun City, Surprise, and parts of Glendale and Peoria), the northeast valley (Paradise Valley and north Scottsdale) and in the east valley (Apache Junction, east Mesa).(Photo: ASU School of Earth and Space Exploration)

Parts of metro Phoenix are sinking by about three-quarters of an inch a year, according to new research by Arizona State University.

The culprit: large amounts of groundwater pumped years ago.

Scientists at ASU’s School of Earth and Space Exploration say ground-elevation levels in Apache Junction are seeing the fastest drop. Sun City West, Peoria and the north Valley are also descending.

People shouldn’t panic, said ASU researcher Megan Miller, co-author of the study published recently in the Journal of Geophysical Research.

“If anything this is slow. It’s rarely going to cause anything you would associate with a disaster. It can be a nuisance but has the potential to cause costly structural damages, and is something to keep an eye on,” she said.

The study didn’t examine whether people in the affected areas are seeing an impact.

If the trend continues over several years, more cracks in the ground called fissures will develop, she said.

Fissures can threaten canals, utility lines, water mains, storm drains and sewers. The foundations of homes and buildings can be damaged as ground levels drop. Changes in ground level also can affect where flood waters flow as water typically seeks the lowest spot when floods occur.

State officials have been aware of what’s called “land subsidence” — where the earth collapses and drops —for years.

The Arizona Department of Water Resources is working with NASA to collect radar data to compliment the department’s data and maps on where land has subsided. The department has been collecting and processing data since 2002 to monitor land subsidence, which is occurring over 2,800 square miles in Arizona.

The department says land subsidence has been happening in Arizona since the early 1900s with parts of Maricopa and Pinal Counties subsiding more than 18 feet since then. In Arizona, land subsidence in so-called geographical basin areas like the Valley is usually due to a lowered water table, according to the department.

But not all areas of the Valley are sinking, the ASU study found. Parts of Scottsdale, Chandler and Mesa have risen by as much as half a centimeter. ASU scientists say they did not observe a change in most of the city of Phoenix.

Miller said the variations of subsidence around the Valley depends on the composition of aquifer layers, the layer thicknesses and bedrock structure, as well as how much groundwater was removed.

When water was pumped out, the sediment layers essentially resettled after breaching a certain level of stress, leaving less available space for water than before and causing the ground level above to drop.

The study attributes the dropping water levels to water pumped from subsurface aquifers before 1980. Legislation passed in 1980 reduced the amount of groundwater pumping, with much of the Valley relying on the Central Arizona Project canal for surface water.

But even with the reduced groundwater pumping — and subsequent increase in the groundwater level — research published in 2005 and 2011 found the ground continued sinking and cracking in parts of the Phoenix metro area and other locations, including Tucson, Casa Grande and Eloy.

In the Valley, fissures have been reported in places including Apache Junction, Queen Creek, Chandler and Scottsdale. Some Valley homeowners have even filed claims and lawsuits against real-estate agents and builders, hoping to be compensated for property damage from fissures they say they weren’t told about.

The ground sinking is not unique to the Valley. It’s also occurring in southwestern Arizona and agricultural valleys in California.

Land subsidance also has been identified in Denver, Colo., the New Jersey coast, Savannah, Ga., and New Mexico’s Albuquerque Basin. The U.S. Geological Survey has identified more than 17,000 square miles of land subsidence in 45 states, an area equivalent to the size of New Hampshire and Vermont combined.

Once the resettlement of the layers, or compaction, occurs, there’s nothing scientists can do to stop or reverse it, ASU’s Miller said.

“It’s important we, as scientists, get a better understanding of what’s happening,” she said, “so we can get a better idea of what the effects will be if we have to change our pumping rates or if we withdraw more water.”

The Bureau of Reclamation has projected about a 1-in-3 chance that as a result of the prolonged Southwestern drought Lake Mead will drop low enough to force Arizona to forgo some of its usual Colorado River water deliveries. The bureau has also forecast a better than a 2-in-3 chance that it will happen in 2017. The agency plans to release a new 24-month projection on Monday.

Any water shortage will initially affect central Arizona farmers, but a prolonged or deepening cut in supplies could force the state to start drawing water from its underground storage.

If Phoenix is forced to increase groundwater pumping due to the drought, that could affect both the extent of land subsidence and the rate at which it occurs, Miller said..

The ASU study used satellite data dating back to 1992 to examine elevation levels around the Valley and compare changes over time.

Miller and the study’s co-author, ASU professor Manoochehr Shirzaei, plan to continue their research, including a model to predict where fissures in the ground could form.

Their research group, the Remote Sensing and Tectonic Geodesy Laboratory, or RaTLaB for short, uses remote sensing to observe and model deformation in the ground due to natural processes: subsidence, volcanic activity , earthquakes and landslides.